3,4-(+)-Methylenedioxyamphetamine (MDA) and 3,4-(+)-methylenedioxymethamphetamine (MDMA) are analogs of amphetamines that are toxic to rodent and primate serotonergic neurons. The mechanism underlying the neurotoxic effect is not well defined, however toxicity may be due to the production of toxic metabolites. In recent studies, several investigators have identified putative MDMA/MDA metabolites, which damage serotonergic and dopaminergic neurons. However, very little is known about the mechanism by which MDMA/MDA metabolites induce neurotoxicity or the mechanism(s) by which these metabolites gain entry into the neuron to produce their deleterious effects. Presumably, these metabolites enter the neuron through the serotonin transporter (SERT). This assumption is made because MDMA binds to SERT and SERT inhibitors such as fluoxetine and fluvoxamine are able to attenuate the toxic effects of MDMA. However, no studies have been conducted which conclusively demonstrate that MDMA/MDA metabolites interact with the serotonin transporter. Therefore, the primary objective of this proposal is to determine whether the toxic metabolites MDMA/MDA interact with the serotonin transporter. Moreover, this project will also determine if polymorphic forms of the promoter region of SERT predisposes cells to varying susceptibility to MDMA toxicity. These objectives will accomplished by (1) demonstrating that MDMA metabolites bind to SERT, (2) demonstrating that metabolite-induced toxicity is mediated through SERT and (3) demonstrating that cells over expressing SERT are more susceptible to MDMA and MDMA-metabolite toxicity. The clinical application is two-fold, the proposal will further elucidate the mechanisms by which MDMA produces neurotoxicity and provide insight into which individuals may be at risk of developing MDMA-induced toxicity.